"Simple Secret Box III: Another Coin Bank" is another coin bank with a secret locking mechanism. Like my Apple Coin Bank design, this model was designed to teach the grandkids to save money, but includes an additional feature; if they change their minds and wish to make an "early" savings withdrawal, well, I didn't shown them how to open the bank. Featuring dovetail fixed joinery and a pin locking mechanism, the trick to opening the box is, well, you'll figure it out eventually...

I've included two tops for this model, "Top.stl" and "Top With Coin Slot.stl", so that you may use the box as either a plain or coin box.

As usual, I probably forgot a file or two or who knows what else, so if you have any questions, please do not hesitate to ask as I do make mistakes in plenty.

Designed using Autodesk Fusion 360, sliced using Cura 3.4.1 and printed in PLA on both an Ultimaker 2+ Extended and an Ultimaker 3 Extended.

I purchased a 3.2mm (.125 or 1/8") by 36" brass rod to make four 28 by 3.2mm lock pins. I cut 4 28mm pieces from the rod, then smoothed the ends using a hand held motor tool.

I printed all parts at .15mm with 20% infill and no supports.

Prior to assembly, test fit and trim, file, sand, etc. all parts as necessary for smooth movement of moving surfaces, and tight fit for non moving surfaces. Depending on the colors you chose and your printer settings, more or less trimming, filing and/or sanding may be required. Carefully file all edges that contacted the build plate, especially in and around the dovetail joinery, to make absolutely sure that all build plate "ooze" is removed and that all edges are smooth. I used small jewelers files and plenty of patience to perform this step.

Assembly.

Press one "Side.stl" into one "End.stl" as shown, carefully noting the orientation of each.

Press the second "Side.stl" into this assembly as shown, again carefully noting the orientation.

Slide the four pins into their guides, two each in each "Side.stl", as shown. The pins should very easily slide guides.

Press the remaining "End.stl" into the assembly as shown, then rock the assembly end to end to make sure the pins slide smoothly in the guides.

Carefully move the pins into their guides to provide clearance for "Top.stl", then place "Top.stl" onto the assembly as shown. Rock the assembly end to end to make sure the pins slide in their guids with ease. To remove the top, place the assembly on a flat / smooth surface and give it a spin as shown in the video, then remove the top.

Press the assembly fully into "Base.stl".

That's how I printed and assembled "Simple Secret Box III: Another Coin Bank".